Abstract. The Global Navigation Satellite System (GNSS) Occultation Sounder (GNOS) is one of the new-generation payloads on board the Chinese FengYun 3 (FY-3) series of operational meteorological satellites for sounding the Earth's neutral atmosphere and ionosphere. FY-3C GNOS, on board the FY-3 series C satellite launched in September 2013, was designed to acquire setting and rising radio occultation (RO) data by using GNSS signals from both the Chinese BeiDou Navigation Satellite System (BDS) and the US Global Positioning System (GPS). So far, the GNOS measurements and atmospheric and ionospheric data products have been validated and evaluated and then been used for atmosphere- and ionosphere-related scientific applications. This paper reviews the FY-3C GNOS instrument, RO data processing, data quality evaluation, and preliminary research applications according to the state-of-the-art status of the FY-3C GNOS mission and related publications. The reviewed data validation and application results demonstrate that the FY-3C GNOS mission can provide accurate and precise atmospheric and ionospheric GNSS (i.e., GPS and BDS) RO profiles for numerical weather prediction (NWP), global climate monitoring (GCM), and space weather research (SWR). The performance of the FY-3C GNOS product quality evaluation and scientific applications establishes confidence that the GNOS data from the series of FY-3 satellites will provide important contributions to NWP, GCM, and SWR scientific communities.

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With the Fourier analysis method, the daytime ionosphere total electron concentration (TEC) is investigated by using global GPS data series from IGS in a high solar activity year (2000). The result shows that the annual variations of daytime TEC are strong in the middle latitudes for the northern and southern hemispheres, and weak in the low latitudes and equatorial region. The amplitudes of semi-annual variations are much larger in the 'far-pole' region (far away from the northern or southern magnetic pole, i.e. northeastern Asia and South American) than those in the 'near-pole' region (North America and Australia). Further studies also show that daytime TEC is maximized at equinox days in most regions, and maximized in winter in the northern 'near-pole' region. In South America and most of Australia, daytime TEC reaches its max value in summer.The atomic/molecular ratio [O/N2] in the neutral atmosphere calculated by the MSIS90 model is also analyzed with the Fourier method. The results indicate that the annual amplitudes of the [O/N2] ratio are large in the northern and southern middle and high latitudes, and the winter anomalies are noticeable very much. According to Rishbeth's opinion, we suggest that the [O/N2] may have important contribution to the TEC annual variations, as well as to the winter anomaly which prevails in the 'near-pole' region. As for the daytime TEC semi-annual variations, we should comprehensively consider the contribution of [O/N2] and the electron production rate associated with the solar zenith angle.